The generator stator core is the largest single component in the train of a turbine generator set. The stator cores are generally manufactured from thousands of laminations of relatively thin steel plates which are stacked, pressed and clamped together into a large cylindrical form, i.e., the shape of the stator core. The clamping is necessary to accommodate variations in thickness (commonly referred to as "crown") of the stock steel plates from which the laminations are punched. Improperly clamped laminations can result in plate vibration during generator operation, which results from magnetic impulses or core elliptical dilation. Moreover, air space between the laminations leads to high thermal resistance and decreased cooling efficiency. Fillers are often inserted into the stack of plates to compensate for voids caused by plate crown. Additionally, the fillers ensure that the clamping pressure is evenly distributed over the full plate area.
Typically, the stator core is assembled from the steel plates directly at the final installation site. However, the large size of the stator core and the need for proper clamping results in stator core manufacturing difficulties, including generous floor space and high crane requirements. Traditionally, two assembly procedures have been employed to form the cylindrical shaped stator core: In one procedure, the steel plates are stacked directly in a stator frame; in the other procedure, the steel plates are first stacked and clamped in an external stacking fixture. The complete stator core is then lifted into the stator frame via a large crane.
The manufacture of stator cores via the traditional methods results in manufacturing lead time and other associated manufacturing difficulties. For example, if the core is stacked directly in the stator frame, the frame must be delivered to the site before any manufacturing steps can occur. Additionally, intermediate core pressing equipment is needed to press and clamp the steel plates together at incremental lengths. If, on the other hand, the stator core is manufactured in an external fixture, the frame does not have to arrive on site before the manufacturing; however, the external fixture itself adds to the manufacturing costs and requires additional floor space on site. Moreover, the external fixture method requires a heavy duty crane of sufficient height to lift the assembled core into the stator frame. In either traditional manufacturing procedure, the core stacking process requires several days to complete.
In addition to assembly difficulties, stator cores assembled according to traditional methods experience operational problems. Such cores have a tendency to settle or relax during service. To help alleviate this tendency, various consolidation techniques and high clamping forces are required during assembly, further increasing the assembly time and costs. Moreover, heavy structural members are required at the core ends to hold the laminations in place, and access for future retightening may be required.
Thus, there is a need for an improved stator core design and manufacturing technique that increases the operational stability, while decreasing the time and cost of manufacturing a stator core.